Ability to skip version check

[martin-braun]

Just the ability to skip the Firefox version check (installed, correct version, etc.), so browsh could be used with any Firefox forks, incl. those with unique version schemas.

Using such flag should also notify the user that it's essentially an unsupported mode. It can only work with firefox.path is provided, because otherwise it tries to resolve the binary path to firefox using which firefox (unix) or registry lookup (Windows).

Ref: #506